1. Field of the Invention
The present invention relates to a driving apparatus and method of driving a liquid crystal display, and more particularly, to a driving apparatus and method of driving a liquid crystal display that improves the brightness of the liquid crystal display in accordance with a back light sequential driving system.
2. Description of the Related Art
The liquid crystal display (LCD), is light weight, thin, and low in power consumption. As a result, LDCs have been increasingly applied in a wide variety of applications including office automation instruments, and audio/video devices. The LCD displays a desired picture on a screen by controlling the transmissivity of light beam in accordance with a video signal applied to a plurality of control switches arranged in a matrix.
The LCD with such a configuration has been replacing the cathode ray tube (CRT) due to the above mentioned light weight and low power consumption. One of the reasons facilitating the increased use of LCDs is technological innovation such as the picture quality improvement of the LCD. Though the cathode ray tube CRT uses an impulse of light emission by the scan of an electron gun, the LCD uses a hold-type of the light emission employing a back light system where a linear lamp (fluorescent lamp) is an illuminating light source. As a result, it is impossible to display a perfect moving picture. In other words, when a moving picture is displayed by the LCD, moving picture contour deterioration occurs due to the hold characteristic, thereby causing deterioration of picture quality.
FIG. 1 is a simulation diagram illustrating a mechanism of how a moving picture contour deterioration is generated when the moving picture is displayed on the display device such as LCD having a hold property. FIG. 1(A) illustrates that a white image being moved in an direction of arrow A is displayed on part of a black background of the LCD. FIG. 1(B) is an enlarged diagram of the boundary area of the black/white images. FIG. 1(C) is a diagram explaining the cause of occurrence of the moving picture contour deterioration. FIG. 1(D) is an enlarged diagram representing the moving picture contour deterioration. Wherein each of squares shown in FIG. 1 represents a pixel. Further, the moving picture contour deterioration is indicated as “a blurring” or as “a moving picture blurring” in FIG. 1.
As illustrated in FIG. 1(C), where one row of the black/white boundary area of FIG. 1(B) is displayed in a time series, a line of sight moves along arrow B, which is slantingly drawn from top left to bottom right, as a displayed picture is moved in an arrow A direction. The brightness of a pixel is sustained or held while a display of one frame is moving. Because the brightness is represented by the integration of the brightness of pixel, the moving picture contour deterioration occurs as illustrated in FIG. 1(D).
On the other hand, such moving picture contour deterioration does not occur in the impulse type of cathode ray tube “CRT”. More specifically, FIG. 2 is the same simulation diagram as FIG. 1(C) where the moving picture is displayed in the CRT not having a hold property. Because the pixel is not displayed while the picture moves between frames, even though the line of sight moves along the arrow B in accordance with the movement of a display picture in an arrow A direction, there occurs no moving picture contour deterioration. In other words, in the impulse type of CRT, black data is displayed between an initial frame and a new frame, so the display picture gets visually vivid due to the black data.
Accordingly, as illustrated in FIGS. 1(C) and 2, an observer's perceived image in the moving picture is vividly displayed in the CRT. As compared with this, the displayed picture becomes blurred in the LCD because of the hold property of liquid crystal in moving pictures. The difference of such a perceived image results from the integration effect of image that temporarily lasts in the eye pursuing the movement. Accordingly, even though the response speed of the LCD is fast, the observer sees a blurred screen owing to a discord between eye's movement and a static image of each frame.
Accordingly, there is a back light sequential driving system for an LDC employing a direct back light where a plurality of lamps are arranged horizontally to prevent the moving picture contour deterioration. The LCD according to the back light sequential driving system turns on/off a plurality of lamps in synchronization with the start time of the scan signal of the display picture, and in addition, when the brightness signals of the same level are applied, the display brightness of the LCD makes the time integration value of the brightness value equalized between each frames, thereby preventing the moving picture contour deterioration from occurring when displaying the moving picture similar to that of an impulse type light emission such as the CRT.
Referring to FIGS. 3 and 4, the driving apparatus for the liquid crystal display employing a back light sequential driving system includes an LCD panel 2 having TFTs at intersection areas where data lines and gate lines cross, a data driver 4 for supplying data to the data lines of the LCD panel 2, a gate driver 6 for supplying gate pulses to the gate lines of the LCD panel 2, a back light unit 10 for providing a light beam to the LCD panel 2 by sequentially driving a plurality of lamps 30, a lamp driver 12 for controlling the back light unit 10, and a timing controller 8 for controlling the data driver 4 and the gate driver 6 as well as driving the lamp driver 12.
A back light unit 10, as shown in FIG. 4, includes a plurality of lamps 30, a lamp housing 22 enclosing the plurality of lamps 30, and a diffusion plate 20 which covers the front of the lamp housing 22. The plurality of lamps 30 is sequentially driven in response to the control of the lamp driver 12. The lamp housing 22 encloses the plurality of lamps 30 and directs the light beam from the plurality of lamps 30 toward the diffusion plate 20 using a reflection surface 24. The diffusion plate 20 allows the light radiated from a plurality of lamps 30 to proceed to the liquid crystal display panel 2 with a wide angle of incidence. The diffusion plate 20 uses a member coated on both sides with films of transparent resin to achieve optical diffusion.
In the LCD panel 2, a liquid crystal is injected between two glass substrates. The TFTs are formed at the intersection areas of the data lines and the gate lines of the liquid crystal display panel 2, thereby providing the liquid crystal cell with the data on the data line in response to a scanning pulse from the gate driver 6. The source electrode of the TFT is connected to the data line, the drain electrode is connected to a pixel electrode of the liquid crystal cell, and the gate electrode of the TFT is connected to the gate line. The liquid crystal display panel 2 is stacked on the diffusion plate 20 of the back light unit 10.
The timing controller 8 rearranges the digital video data supplied from a digital video card (not shown) by red (R), green (G), and blue (B), respectively. The data (R, G, B) rearranged by the timing controller 8 is supplied to the data driver 4. Further, the timing controller 8 generates a data control signal and a gate control signal using the horizontal/vertical synchronization signal (H/V) input to itself. The data control signal including a dot clock (Dclk), a source shift clock (SSC), a source enable signal (SOE), and a polarity inversion signal (POL) is supplied to the data driver 4. The gate control signal including a gate start pulse (GSP), a gate shift clock (GSC), and a gate output enable (GOE) is supplied to the gate driver 6. Further, the timing controller 8 controls the lamp driver 12 so that the back light unit 10 may sequentially be driven at a point of time when the data is completely supplied to the liquid crystal cell.
The data driver 4 latches the sampled data line by line after sampling the data in accordance with data control signal from the timing controller 8, and then converts the latched data into an analog gamma voltage from a gamma voltage supplying part (not shown). The gate driver 6 includes a shift register for generating the gate pulse sequentially in response to the gate start pulse (GSP) among the gate control signal from the timing controller 8, and a level shifter for shifting the voltage of the gate pulse to the voltage level suitable for driving the liquid crystal cell. The lamp driver 12 drives a plurality of lamps 30 of the back light unit 10 sequentially in response to the lamp driving control signal from the timing controller 8. More specifically, the lamp driver 12 drives a plurality of lamps 30 sequentially after the data voltage is supplied to the liquid crystal cell completely.
In the driving apparatus of the liquid crystal display device as described, a plurality of lamps 30 are driven sequentially when a plurality of gate lines are driven during one frame as shown in FIG. 5. More specifically, when the gate pulse is supplied to the gate lines of at least GL—1 to GL—1+M among N gate lines and the data voltage is completely supplied through the data lines to the liquid crystal cell, the first lamp 30 is turned off after being turned on. Further, when the gate pulse is supplied to the gate lines of at least GL—(1+M)+1 to GL—1+2M among the N gate lines and the data voltage is supplied through the data lines to the liquid crystal cell, the second lamp is turned off after being turned-on.
Here, the computation of the brightness is explained according to the above described scanning back light driving method. In the first place, the brightness of hold-type back light driving method constantly turning on back light is defined as equation 1. Here, it is assumed that the brightness of 1 frame is 1 in case that one lamp is turned on.                                                                         Brightness                (                                  Hold                  ⁢                                      -                                    ⁢                  Type                                )                            =                            ⁢                                                                    (                                          1                      +                      1                      +                      1                      +                      ⋯                      +                      1                                        )                                    /                  1                                ⁢                                                                  ⁢                Frame                ⁢                                                                  ⁢                Time                                                                                        =                            ⁢                                                n                  /                  1                                ⁢                                                                  ⁢                Frame                                                                        [                  Equation          ⁢                                          ⁢          1                ]            
Hereby, the brightness of the scanning back light driving method is reduced in inverse proportion to the number of the lamps by contrast with the hold-type back light driving method as illustrated in equation 2.                                                                         Brightness                (                                  Scanning                  ⁢                                                                          ⁢                  Type                                )                            =                            ⁢                                                                    (                                                                  1                        /                        n                                            +                                              1                        /                        n                                            +                                              1                        /                        n                                            +                      ⋯                      +                                              1                        /                        n                                                              )                                    /                  1                                ⁢                                                                  ⁢                Frame                                                                                        =                            ⁢                                                1                  /                  1                                ⁢                                                                  ⁢                Frame                                                                        [                  Equation          ⁢                                          ⁢          2                ]            